1. Field of the Invention
The present invention relates to fasteners and, in particular, to fasteners having a body which, when inserted into a workpiece, is deformed by the addition of a second member, thereby preventing its removal from the workpiece.
2. Prior Art
To join a workpiece such as a placard or sign to a second workpiece such as a wall, or to join two planar workpieces together, it is well-known to utilize a blind rivet as a fastener. For example, U.S. Pat. No. 3,198,058 discloses a blind rivet consisting of a body having a flared head which forms a flange to clamp workpieces together, a split shank, and a central bore which intersects the split shank. The rivet also includes a pin having a slightly flared head and a chamfered tip.
The body is inserted into predrilled pilot holes in the workpieces until the flanged head abuts the face of one of the workpieces. The pin is driven into the bore until the slightly flared head of the pin abuts a mating countersink in the bore. The chamfered tip of the pin engages sloping shoulders formed in the split end of the body and forces the ends apart, thus clamping the workpieces between the split ends and the flanged head of the body.
The pin includes a flange extending about its periphery and located about midway along its length which prevents the pin from being removed from the bore of the rivet. However, the rivet can be removed from the workpieces simply by driving the pin entirely through the bore of the rivet and out the other side of the workpieces. The body of the rivet then can be removed from the holes.
Other types of rivets include bodies having barbed or threaded exteriors on their shanks to prevent removal of the rivets once mounted. For example, U.S. Pat. No. 3,461,772 discloses a plastic rivet having a shank with a series of barbs on its external periphery. The body of the rivet includes a flared, flat-faced head and the body of the rivet is inserted into a predrilled countersunk pilot hole so that the flat-faced head is flush with the surface of the workpiece. A pin having a series of external ridges and a slightly flared head is inserted into a central bore, thereby driving the body of the plastic rivet apart so that the barbs engage the sides of the predrilled hole. The body includes a longitudinal slit which extends transversely across the shank so that the shank terminates in a pair of legs which can be driven apart from each other by the pin into the walls of the hole.
Other types of fasteners are of the anchor-type. For example, U.S. Pat. Nos. 3,937,122 and 3,022,701 disclose anchors having bodies with ribbed or threaded shanks and central bores sized to receive a pin which includes external threads. In each of these patents, the pin itself has a slotted head so that it may be inserted or removed by the use of a screwdriver.
In many applications, it is desirable to utilize a blind rivet or anchor which will permanently join the workpieces together or become permanently embedded in a single workpiece. For example, such an application may involve the mounting of a street sign or other directional sign to a building or other support so that it may not be removed or vandalized. Another area of usage would occur in correctional facilities or jails where it is desirable permanently to mount protective screens over windows and other openings to prevent the escape of the individuals detained in the facility.
A disadvantage of the aforementioned fasteners and anchors is that it is relatively easy to remove the pin from the body of the anchor or rivet, then remove the rivet itself from the hole within which it is mounted. Removal of the anchor or rivet is especially easy if a portion of the rivet body and/or pin extends above the surface of the workpiece within which it is mounted. Similarly, if the central pin includes a slot to receive the beads of a screwdriver, it is relatively easy to unscrew the pin from the body of the rivet or anchor and then remove the anchor.
Those anchors or rivets which have heads that do not protrude a great distance from the surface of the workpiece, or in which the pin does not contain a slot head or does not protrude from the surface of the workpiece or body of the rivet, may still be easily removable and therefore undesirable for use in such security environments. With this latter type of fastener, the pin may often be removable by driving it completely through the body of the fastener and out the other side of the workpiece. It is then a simple matter to remove the body of the fastener from the hole in the workpiece.
Accordingly, there is a need for a fastener which may be used in high-security areas or to mount signs so that they may not be removed or vandalized which can provide a permanent and tamper-proof means of mounting. There is a need for a fastener which does not require a flanged head which protrudes outward from the surface of the workpiece and thus presents a vulnerable portion of the fastener; and a tamper-proof fastener in which the central pin cannot be removed, unscrewed, or driven through the fastener to the other side of the workpiece. Such a fastener must also be relatively inexpensive to manufacture and must have a simple design which lends itself to mass fabrication. Furthermore, such a fastener must be relatively easy to install and not require expensive power tools to secure the fastener to the workpiece.